1. Field of the Invention
The present invention relates to a substrate having an insulating base and can be applied to, for example, a substrate having a ceramic or glass ceramic base.
2. Description of the Background Art
In wide-raging areas of technology such as information network, ceramic substrates and glass ceramic substrates are heavily used.
A ceramic substrate is a complex consisting of a ceramic base, a conductive portion, a plating film and the like. The conductive portion is formed on the ceramic base and the plating film is so formed as to cover the conductive portion. When the ceramic substrate having the above structure is manufactured, there is a case where part of the plating film is joined to an upper surface of the ceramic base.
When the ceramic substrate having the above structure is thermally treated with temperature changes, there is a case where cracks appear on the upper surface of the ceramic base near the conductive portion. This is caused by a large difference between the coefficient of thermal expansion of the plating film and that of the ceramic base.
Specifically, in the case where part of the plating film is joined to the ceramic base, when the ceramic substrate is thermally treated, a stress is produced between the plating film and the ceramic base due to the difference in coefficient of thermal expansion. Since the difference in coefficient of thermal expansion is large, the stress which is thereby produced becomes large and as a result, it causes cracks in the ceramic base.
For the purpose of suppressing occurrence of such cracks, there are techniques of changing manufacturing processes such as lowering the temperature for the thermal treatment and reducing the degree of temperature changes in the thermal treatment. There are other techniques of changing a pattern of the conductive portion, for example, by rounding comers of the conductive portion or reducing the density of formation in the conductive portion.
Patent Document 1, Japanese Patent Application Laid Open Gazette No. 3-209793, shows another technique in relation to the present invention.
In adopting the above techniques of changing the manufacturing processes, a free design of processes becomes impossible and moreover the process time becomes longer. As a result, due to the above problems in the manufacturing processes, the production efficiency of ceramic substrates becomes lower.
In adopting the above techniques of changing a pattern of the conductive portion, there arise problems of requiring a long time to change a design and increasing a manufacturing cost. Further, since there is no quantitative rule for design, the design change of the conductive portion is dependent only on experience. This requires setting of a larger design margin, resulting in lower efficiency of development.
Further, in order to reduce the stress applied onto the base, a method of reducing the thickness of nickel-plating film may be used. When components are soldered to the conductive portion, however, the nickel-plating film serves as a main junction layer. Therefore, in order to ensure good soldering, thinning of the nickel-plating film or simplification should not be easily made.
The cracks appearing in the upper surface of the ceramic base has an ill-effect on reliability and durability of the ceramic substrate. Therefore, suppressing occurrence of such cracks may allow significant cost reduction and contribute to an increase in fields of application.